The invention relates to a resonance label and to a process for the production thereof.
It is known to use resonance labels for providing protection from theft in department stores, supermarkets and the like. Such are attached to the goods to be protected and have to be deactivated at the checkout to avoid triggering a burglar alarm at the exit. For deactivation, a strong surge current is induced in the resonance label which constitutes an oscillating circuit, the said surge current virtually producing a short-circuit (for example according to U.S. Pat. No. 3,624,631). This requires enormous fields which on the one hand, in view of the general proximity of the checkout and exit, may lead to undesired mutual effects and to false alarms and, on the other hand, is also energy-consumptive. Weaknesses in the circuit tracks also have an adverse effect on the quality Q of the oscillating circuit.
In the past, attempts have therefore been made to reduce, as far as possible, to reduce the energy required for deactivation. This is achieved either by a complicated label structure having two different oscillating circuits (for example according to U.S. Pat. No. 3,810,147) or by another obvious measure (which was also taken for other reasons) and which is comprised of making the insulating layer, which is generally identical to the base, as thin as possible. However, the aforementioned approaches have their limits.
To further reduce the energy required for deactivation, U.S. Pat. No. 4 567 473 proposes introducing a notch at one point, in particular in the region of the capacitor plates, after production of the label. However, introducing such a notch of predetermined depth into a substrate which is only a few hundredths of a millimeter thick is an extremely difficult undertaking. In fact, highly variable coil qualities are obtained in practice, since either the conducting layer is drawn to varying depths into the insulating layer with the cut or--since the insulating layer is under tension during the process--left the surface of the incision exposed, so that now a higher deactivation energy is actually required in order to, so to speak, draw a spark around the corner.
Hence, there is on the market another embodiment in which the label, after its production, is subjected to localized pressure in the region of the conducting layers which are opposite one another and separated only by the insulating layer, so that the dielectric becomes thinner in localized areas as a result of the compression. This is generally carried out using an embossing roll which compresses an area of the magnitude of a square millimeter. This has further disadvantages. Fluctuations in the thickness of the various layers of the manufactured label have a considerable effect on the reduction in thickness of the dielectric finally achieved. If the distance is too large, the deactivating field is not sufficient to fulfill its purpose. However, if the distance is too small, the label may be deactivated during recognition in the system. Furthermore, the relatively large thinner area is sufficient to have a strong influence on the resonance frequency of the oscillating circuit, making it more difficult to set the resonance frequency to a predetermined value or at least to a narrower frequency range. This situation is further aggravated by the fact that the only dielectric used in such labels to date was polyethylene, whose thickness is relatively large and in the range from about 26-30 .mu.m, and which is to be brought to 5-8 .mu.m by the embossing procedure.
Although other materials, such as polystyrene or polypropylene, are thinner and would in principle be more suitable for capacitor formation, they have a very small dielectric loss factor which, particularly in this known type of label, leads to problems with deactivation.
In summary, it may thus be stated that all attempts to date to reduce the field strength required for deactivation, by means of selective reducing of the thickness of the insulating layer, have been unsatisfactory since they have led to hither rejection rates. However, the above description of the prior art also shows that substantial attempts to overcome the problem have been made without success.